1. Field of the Invention
The present invention relates to a continuous thru-wire welding machine and, more particularly, to a highly simplified welding machine having a repeatable, accurately controllable welding force.
2. Description of the Prior Art
Complex electronic assemblies include large numbers of closely spaced, miniature terminals which must be electrically interconnected. This is achieved simply and conveniently by the use of a continuous insulated wire, typically nickel coated with teflon. As is known in the art, the continuous insulated wire is snaked to individual terminal locations without the need of removing the insulation prior to assembling the wire for welding. Rather, advantage is taken of the cold-flow properties of teflon.
Specifically, a pair of elongated conductive electrodes having adjacent ends are mounted for movement toward the terminal. Either the electrodes can be concentric with a dielectric therebetween or the electrodes can be positioned in coaxial, spaced relationship and movable toward and away from each other. Typically, one electrode is hollow and the wire is continuously fed therethrough. When a connection is desired, the electrodes are moved into contact with the terminal. Pressure is applied between the electrode and the terminal to break through the teflon insulation. Thereafter, an electrical pulse is applied to the electrodes, which pulse causes a diffusion bond to occur at the junction of the wire and the terminal. The electrodes are then positioned consecutively to all of the other common points in a given signal string.
During such a process, it is desirable and, in fact, necessary, to accurately control the force on the wire positioned between the terminal and the electrodes. That is, if the force is too low, one does not achieve cold-flow of the teflon insulation and a good bond cannot be achieved. If the force is too high, the electrode will damage the wire. The result is a necessity to control the force within narrow limits. In the past, welding machines have utilized complex and expensive pneumatic, hydraulic, or mechanical means for varying the pressure between the welding electrodes and the terminal. Most importantly, these machines have proven incapable of providing a repeatable, accurately controllable welding force.